Wireless communications have become a major way of communications and in many applications are replacing conventional land-based communication systems. There are many kinds of wireless communication systems, such as the cellular phone system, the wireless LAN, and the WiFi as shown in FIG. 1. The most commonly used system is the cellular system, and the other wireless communication systems are growing rapidly.
One common problem frequently encountered in wireless communications is the presence of interfering signals transmitted by devices other than the particular sender with which the receiver wants to communicate. Depending on the types of the wireless communications, the interferences may be intentional, such as the jamming of military wireless transmissions, or accidental and resulting from having multiple users who are sharing a common wireless channel with or without base station. Interferences are found in various common wireless communication systems, and the users of a wireless channel may be, for instance, a cellular phone, a laptop computer, etc.
The presence of interfering signals can severely compromise the ability of the receiver to discern the signal from the intended sender, resulting in a significant reduction of the effectively channel capacity for the wireless transmissions from the sender to the receiver. In the cellular system, multi-user technologies have been used to combat the inter-user interference by a base station. The three commonly used multi-user technologies are the Frequency Division Multiple Access technology (FDMA), the Time Division Multiple Access technology (TDMA), and the Code Division Multiple Access technology (CDMA). In FDMA, the users are assigned non-overlapping frequency slots (by the base station), and hence the inter-user interference can be avoided. Similarly, in TDMA the users are assigned non-overlapping time-slots, and in CDMA users are assigned non-overlapping orthogonal codes. Because the power limitation of the base station, the area of the cell it controls is limited to its neighborhood. Therefore, any user outside the cell can not be controlled by this base station, and hence may interfere with the users in the cell.
Interferences in wireless transmissions are also a serious issue for Wireless Local Area Network (LAN). A wireless LAN uses part of the frequent spectrum that is free to everybody and hence it costs nothing to use the spectrum. It uses the internet to reach the outside world and hence it again costs nothing. However, the interferences between the users can be significant because this part of spectrum is unregulated. This is one of the major challenges for wireless LAN. The current technology confines it to be in a local area with limited users. It does not allow it to be developed to reach its full commercial potential value. WiFi is one kind of Wireless LAN and hence shares the same advantages and challenges. Interferences between the users also exist in ad hoc network, a closed wireless communication network but without a central commend.
In view of the foregoing, there is a significant need for a way to effectively counter the negative effects of interfering signals and to enhance the channel capacity of wireless communications between a sender and a receiver in the presence of interfering signals.